Persons who are deaf or hearing-impaired who cannot hear well enough to use the telephone commonly make use of communication terminals specifically constructed and designed to enable such persons to converse over the telephone lines. Such devices are referred to as telecommunication devices for the deaf or TDD and include both a keyboard and a display connected to the telephone through a modem (modulator/demodulator). The modem is typically built into the TDD and either directly wired to a telephone line or coupled through an acoustic coupler to a normal telephone handset. The TDD is capable of transmitting information over a telephone line by means of coded tones to another similar TDD connected at the opposite end of the telephone line through another modem.
There are several protocols by which electronic devices are capable of communicating through analog lines, such as telephone lines. The most common used in the industry is referred to as ASCII (American Standard Code for Information Interchange), and is commonly used for information interchange between computers. However, historically, TDD have operated on a different protocol, referred to as Baudot. There is both a Baudot code and a protocol of communication referred to as Baudot/Weitbrecht.
The Baudot/Weitbrecht communication protocol has several idiosyncrasies which result from the early equipment used to implement it. Baudot devices communicate at 45.5 Baud and utilize a 5 bit code utilized only by Baudot machines. In the Baudot code transfer protocol, the presence of a logical 1 is indicated by an 1400 Hertz tone while the presence of a logical 0 is indicated by an 1800 Hertz tone. Under Baudot code, where no character or bit is being transmitted, there is no tone transmitted on the line. Baudot machines operated in simplex, that is to say if two Baudot machines are communicating, only one is capable of transmitting at a time. The normal convention of Baudot communication is that the receiving station communicate in Baudot first to the calling station. The ability to communication to the outside world through a TDD can be an essential, sometimes even vital, service to a deaf person. For example, in times of emergency, a deaf person must be able to use his or her TDD to make an emergency call of a kind that a hearing person makes orally. It is for that reason that many emergency service operators (including 911 services) in the United States are required to be equipped with TDD, and incoming call detectors to sense TDD on the line, so as to handle in-coming calls from deaf or hearing impaired persons in emergency situations. In such situations, however, the existing Baudot protocol presents a deficiency in the operation of a TDD device in such a emergency situation. The deficiency arises because the calling TDD normally waits for a response from the answering TDD prior to transmitting characters. If the TDD user is calling a 911 emergency service center, the person answering the telephone may hear only silence on the telephone line. Several emergency service operating systems have implemented circuits, therefore, intended to monitor the incoming lines and detect the characteristic TDD signals (1400 and 1800 Hertz) on the incoming line, and, when such signals are received, to alert the operator that the call is coming from a TDD or to switch that incoming line from a hearing person to a TDD. Thus, many emergency service operator systems are presently equipped with devices, referred to as TDD detectors, which are capable of sensing TDD generated codes and alerting the operator that a call is from a TDD, so that the incoming signal can be connected to the services own TDD.
It has been a problem with such TDD detectors and incoming signals from TDD in that the TDD detectors are capable of being falsely tripped. The 1800 and 1400 Hertz of Baudot tones are sounds which are common in the human environment, making it possible for background sounds to be interpreted as a Baudot character. Also, some TDD detectors may require several characters to be received by the calling TDD, which may leave the calling deaf person in some confusion as to what information has been received by the emergency services operator. Both difficulties would be ameliorated if a protocol existed by which TDD could identify specifically themselves, and the capabilities of their TDD, in a manner that was sufficiently distinctive so as to render extremely unlikely either falsely triggering or not triggering at all the TDD detector present at the emergency services facility.